1. Field of the Invention
The present invention relates generally to the field of ink compositions and more particularly to water insoluble ink compositions which can be printed in a substantially water insoluble form, but which can be cleaned up using aqueous solutions. Although the ink composition of the present invention can be used in a variety of printing processes, such composition has particular applicability to lithographic printing. The present invention also relates to novel resins used in the preparation of such ink compositions and to methods of making and using such compositions and resins.
2. Summary of the Prior Art
A wide variety of printing processes currently exist in the art. Although it is contemplated that the ink composition and related resins will have applicability to most if not all of these processes, it has particular applicability to a lithographic printing processes commonly referred to as lithography. Lithography is a method of printing which relies on differences in solubility and surface wetability between an oil based component and an aqueous or aqueous based component to effectively transfer the printing ink to the desired image area and prevent it from transferring to the nonimage areas.
During conventional lithographic printing, an oil based ink composition and an aqueous fountain solution are applied to a printing plate. Because of the solubility and the wetability differences of the oil based and aqueous compositions, the fountain solution is preferentially attracted to and preferentially wets oleophobic nonimage areas, while the ink is preferentially attracted to and preferentially wets oleophilic image areas. It is well known and accepted in the art that successful lithographic printing requires inks which exhibit stability relative to water and thus remain water insoluble. Absence of this characteristic will result in poor print quality, poor edge definition, dot gain and various other print deficiencies. Thus, lithographic inks are selected and formulated for their ability to remain stable, cohesive and insoluble when contacted with water. The simultaneous feeding of the ink composition and fountain solution to the print plate is accomplished through a variety of methods and roller configurations known in the art.
During the course of lithographic printing, the printing plates are periodically changed as one job is completed and another started. Whenever this occurs, the blanket cylinder in an offset process must be cleaned to remove ink residue which is present from the previous job. Further, if a change of ink is desired, the entire print train including the application rollers, the print plate and the blanket must be cleaned. Such cleaning is commonly accomplished using an appropriately formulated wash solvent. To be effective as a wash solvent, the wash must be compatible with, or be able to dissolve or sufficiently disperse the ink. Since the inks are oil based or water insoluble, this necessitates the use of organic or petroleum based or other non-aqueous wash solvents to effectively remove the ink from the rollers, printing plates, blanket cylinder, etc.
These organic wash solvents can give rise to employee safety concerns and are a large source of both air and water pollution as volatile organic compounds (VOC's) are dispersed into the atmosphere or disposal systems. Such pollution is due to evaporation into the ambient air or into a venting system during the washing of the print components as well as the disposal or laundering of shop towels and rags used in the cleaning process. Attempts to develop water based wash or cleaning solutions have not been successful due to the inherent ability of the ink to resist water. Attempts have also been made to use water/solvent mixtures by emulsifying petroleum-based solvents into water through the use of emulsifiers and surfactants, but these products suffer from inherent instability due to immiscibility of water and the solvents and perform slowly. Further, such mixtures do not completely eliminate the use of petroleum solvents which are still commonly present in an amount of about 30-80%. Other industry trends include the use of solvents such as terpenes. While not petroleum based, their performance has been marginal and they are costly and in short supply. In addition, their use and disposal also pose environmental concerns.
Thus, although offset lithography is recognized and established as a dominant printing process for certain applications, drawbacks exist because of the pollution and disposal problems referred to above. These are becoming more of a drawback and more of a problem as new pollution control regulations and standards are mandated. In fact, because of the very nature of the lithographic process at least one of the components (either the ink composition or the fountain solution) must be oil based or water insoluble. This in turn necessitates the use of an organic or petroleum based solvent to wash the apparatus. This is generally accepted as a necessary limitation of the lithographic process about which little can be done.
For nonlithographic applications, certain water based or water soluble inks are currently available. Some of these utilize water dispersible or soluble resins which have been preneutralized to provide such properties. See for example U.S. Pat. No. 4,966,628 issued on Oct. 20, 1990 to Amos et al. and U.S. Pat. No. 4,154,618 issued on May 15, 1979 to Burke. While these inks can be cleaned up with water or water solutions and thus reduce pollution concerns, it is generally recognized that existing water soluble or dispersible inks exhibit inferior water and alkaline resistance as well as inferior drying, adhesion and gloss properties when compared to conventional solvent based inks.
Accordingly, there is a need in the art to address the above problems, while still providing an ink composition which, when printed, will result in highly acceptable print quality and durability.